teaspoon, teaspoonful,
tablespoon, tablespoonful

Units of liquid and dry capacity mainly used in cooking and to give medicines at
home.

United States

In the United States, a teaspoon is 1/3 of a
tablespoon and is equal to 4.928 922 milliliters. A study several decades ago by the U.S. Bureau of Standards found that the typical silver teaspoon and measuring teaspoon held 1½ fluid drams (about 5.5 mL), not the 1 fluid dram
(about 3.7 mL) often given in reference works.

In a publication of 1975 that anticipated conversion to the metric system, the
National Bureau of Standards said:

There will be no need for much change in our recipes if the new metric
recipes remain volumetric and if, as anticipated, the utensils retain
approximately the same ratio as the customary cup (237 ml), teaspoon (4.9 ml),
and tablespoon (14.7 ml). This is easily achieved by adopting a “metric
cup” of 250 ml (¼ of a liter); a “metric teaspoon” of 5 ml, and a
“metric tablespoon” of 15 ml.1

In fact, by the 1990s almost all the teaspoon measures in stores were made to
contain 5 mL, and tablespoons 15 mL, and many were so marked. This is a
difference that makes no difference, because few cooks measure ingredients with
an accuracy of 0.1 mL.

Great Britain

In Great Britain, a teaspoon is ¼ of a tablespoon, but a spoonful of a
dry ingredient traditionally means a rounded spoonful, with as much heaped above
the spoon edge as lies within it. (A rounded spoonful is not a heaping spoonful,
which would be as much as the spoon could hold.) On this theory, a British teaspoonful
of a dry ingredient would be about 7 milliliters, or a bit short of 1½ U.S. teaspoons.

An English pharmacopeia of 1618 defined the tablespoon as the volume of
distilled water weighing 3 drachms, which would be about 12 milliliters. Pharmacists,
and following them such respected sources as The Economist, say the
British tablespoon = half of an imperial fluid ounce, or about 14.21
milliliters, so that there would be 10 tablespoons in an imperial gill and 20 in
a half pint, and the teaspoon would be about 3.55 milliliters.

Cooks, however, don't seem to agree. Mrs. Beeton, author of the classic 19th-century English
cookbook, says under the heading “Liquid Measure” that a quarter pint
contains “6 large tablespoonfuls.” Since an imperial quarter pint (a
gill) contains 5 imperial fluid ounces, that would make the “large
tablespoonful” 5/6 of an imperial fluid ounce, about 23.7 milliliters, and the
teaspoon about 6 milliliters.